The atomic force microscope (AFM) enables nanomechanical measurements with high spatial resolution. Some AFM imaging modes require the cantilever tip to oscillate in contact with or near the surface. The cantilever oscillation can be actuated with piezoelectric, electrostatic, photothermal, thermomechanical or magnetic schemes.
The first papers reporting magnetic actuation of a microcantilever used a magnetic coating or particle on cantilever and an external AC field. The presence of the magnetic material on the cantilever comes with challenges in fabrication and implementation. Buguin and co-workers proposed an improved Lorentz force actuation scheme, where the magnet was external to the cantilever as described in and U.S. Pat. No. 6,862,923. In this scheme, the actuation was driven by an AC current flowing through a cantilever current loop in the presence of an external magnetic field. Buguin taught the use of a uniform magnetic field oriented substantially perpendicular to the current loop. A related approach by Enders and co-workers analyzed cantilever excitation spectra induced by Lorentz force and studied thermomechanical spectrum at high power. A Lorentz force actuation scheme is market commercially by Asylum Instruments for actuation of soft cantilevers in fluid, called the “iDrive™ Fluid Imaging Option.”
Similarly, U.S. Patent Application Publication US 2011/0126329 discloses a magnetically actuated cantilever chip including a centrally supported cantilever having a magnetic element positioned on the cantilever body at an end opposite to the probe tip. Actuation of the cantilever is accomplished by flowing a current through a loop on the cantilever chip positioned nearby to the magnetic element to induce a local oscillation at the magnetic element. Here both the magnetic elements and current loops are constructed to generate extremely localized interactions to permit independent operation of arrays of cantilevers.
U.S. Pat. No. 6,668,627 discloses a magnetically excited cantilever. Here, the cantilever is placed in a magnetic field and a current is passed through a loop in/on the cantilever to induce vibrations. The cantilever devices include chemically sensitive films for use as highly sensitive chemical sensors. The cantilever devices are non-heating and are designed to limit heat generation in the devices to minimize the impact of heat on and achieve non-interference with the chemically sensitive films.
Within the field of AFM, self-heating AFM cantilever probes have been developed. An example is shown in FIG. 1. These probes are described for example in U.S. Pat. No. 7,497,613, which is incorporated by reference. These cantilever probes 101 have conductive arms 103 and include a resistive heater 107 in the vicinity of the tip of the end 106 of the cantilever structure. When current is directed through the resistive heater, the dissipated power heats the cantilever and tip such that a microscopic region of a sample can be locally heated. The probe tips are very sharp, <20 nm end radius and thus can be used to measure thermomechanical properties of materials on the sub-micron and sub-100 nm length scales. With these probes it has been possible to investigate the nanometer-scale thermal properties of materials and perform nanometer-scale lithography.
Contact Resonance AFM
One measurement mode of AFM is called contact-resonance AFM (CR-AFM). This technique is described in various publications by Yamanaka, Rabe and Arnold, Yuya, Turner and Hurley, Stan and Cook for example, and patents including U.S. Pat. No. 6,983,644. In this mode a cantilever is oscillated with its probe tip in contact with a sample surface. With this arrangement, the cantilever will resonate at frequencies that are dependent on the mechanical properties of both the cantilever and the sample. By measuring shifts in these frequencies, the mechanical properties of a sample can be inferred. A major challenge with CR-AFM has been the method of actuating the cantilever. Most researchers use piezoelectric actuators or ultrasound transducers to excite cantilever resonances. While these can work, they suffer from bandwidth and power limitations, and parasitic resonances. The current inventors found that these parasitic resonances can be particularly problematic while performing measurements of sample mechanical properties as a function of temperature. The reason is that the mechanical properties of a polymeric material can change dramatically as a function of temperature. As such, during variable temperature measurements, it is increasingly likely that a contact resonance frequency of the cantilever can sweep through a parasitic resonance, thus compromising the quality of the measurement.
Dynamic Mechanical Analysis
A very commonly used instrument for characterizing the bulk properties of a material is dynamic mechanical analysis (DMA). DMA works by applying an oscillating stress to a sample and measuring the time-dependent strain, or the strain rate. Analysis of DMA data gives information about material stiffness, viscosity, thermal transitions and activation energies, for example. DMA is a critical and widely used tool to measure the viscoelastic properties of bulk materials, but suffers from extremely slow measurement speed and complete lack of spatially resolved information. Large and growing material classes employ nanoscale composite structures to achieve desired material properties. Conventional DMA cannot measure the local behavior within these heterogeneous materials, producing only information about the aggregate bulk behavior. There is a growing need for instrumentation that can characterize micro and nano-structured materials on the length scales they are engineered.
Some attempts have been made to address this issue, including phase imaging, pulsed force microscopy, nanothermal analysis, and nanoDMA. Phase imaging (U.S. Pat. RE36,488) involves measuring changes in the cantilever oscillation phase while in tapping mode. These variations can illuminate variations in damping, friction, adhesion and elasticity, but cannot easily distinguish or quantify between these effects. Nanothermal analysis (nanoTA) has been commercialized by Anasys Instruments. NanoTA records the cantilever deflection while ramping the tip temperature with self-heated cantilevers. NanoTA can identify glass transitions on some materials by observing changes in slope of the deflection versus temperature curve. It has been successful on many materials, but can fail on some samples, especially those that are highly crosslinked, highly crystalline, and/or very thin, e.g. <100 nm.
A commercial instrument called nanoDMA has been developed by the Hysitron company. It applies an oscillating force to a sample surface using a sharp tip. The maximum oscillation frequency reported by the manufacturer is 300 Hz, roughly similar to conventional dynamic mechanical analyzers (DMA). It also suffers from slow measurement speeds similar to conventional DMA, as discussed below.
The slow measurement speed of DMA and similar bulk measurements results in large part from the macroscopic scale of the heated region of the sample and attendant instrumentation. The sample and often a significant portion of the DMA apparatus is typically enclosed in a large temperature controlled enclosure. Because of the large volume that must be heated and cooled, it is not uncommon to employ heating/cooling rates in the range of 1-10° C. per minute. It is also not uncommon to wait for 30 minutes or more for the system to stabilize at each temperature. Thus to characterize the viscoelastic properties of a sample at a significant plurality of frequencies and temperatures can take hours to days.
Modern materials are also subject to stresses and strains over an extremely wide range of frequencies from static loads up to high vibrations encountered in automotive and aerospace applications, to extreme shocks encountered in impact and ballistic applications. No current tool can rapidly examine the temperature dependent viscoelastic response of these materials on the length scales they are being engineered. Further, no current tool can examine material properties on this length scale and over the wide range of strain rates encountered by modern materials.